1) Field of the Invention
The present invention relates to an image forming apparatus that employs an electrophotographic process to form an image, and to a process cartridge detachably mounted in the image forming apparatus and an image forming method.
2) Description of the Related Art
Digital type image forming apparatuses that employ an electrophotographic process to form images are widely used. Facsimiles, printers, and copying machines are examples of the image forming apparatuses. The image forming apparatus generally includes a photoconductor, a charger, an image exposing device, a developing device, a transfer device, a separator, a cleaning device, a decharger, and a fixing device.
A photoconductive material used for the photoconductor includes zinc oxide, cadmium sulfide, cadmium selenide, an amorphous selenium type material such as a-Se and a-As2Se3, an amorphous silicon type material such as a-Si:H and a-Si:Ge:H, and polyvinyl carbazole. However, these photoconductive materials are hazardous and costly. Therefore, the now a days organic photoconductors (OPC) are used as the photoconductive material because it has many advantages from the viewpoint of energy saving, resources saving, manufacturing easiness, possibility of highly sensitive design, low costs, and non-contamination.
When the organic photoconductor is used, the typical layer structure includes a single layer structure or a dual layer structure (hereinafter, “function separated type photoconductor”). The single layer structure includes a layer of material that is a mixture of a material for generating an electric charge and a material for transporting the generated charge. The function separated type photoconductor includes two distinctly separate layers of the material for generating the electric charge and the material for transporting the generated charge. Of these two types of the photoconductors, the function separated type photoconductor is more easily available in the market.
Because analog type of image forming apparatuses are now being replaced with digital type of image forming apparatuses, photoconductors that can be suitably used in the digital type of image forming apparatuses are being developed.
A typical photoconductor for the digital type of image forming apparatuses (hereinafter, “digital type photoconductor”) includes a base coating layer of thickness ranging from 1 micrometer (μm) to 20 μm, a charge generation layer of thickness ranging from 0.1 μm to 5 μm, and a charge transport layer of thickness ranging from 10 μm to 50 μm in this order on a conductive support made of aluminum or the like.
The charge transport layer formed on the uppermost layer of the photoconductor has an advantage in that the degree of design flexibility to mechanical durability is widened. Polycarbonate resin (A type, C type, Z type, or the like) is generally used for a binder resin of the charge transport layer. When this resin is used for a photoconductor, the number of durable sheets is about 50,000 sheets to 80,000 sheets as the A4-size paper.
The durability of the photoconductor can be increased by various methods. One approach is to use a polymer for the charge transport layer and form a abrasion-resistant protective layer such as an amorphous carbon film or an amorphous silicon film on the charge transport layer by from about 0.5 μm to about 5 μm using a plasma chemical-vapor deposition (CVD) method or a vacuum evaporation method. Other approach is to form a resin layer or a photoconductive layer on the charge transport layer by from about 1 μm to about 10 μm. More specifically, the resin or photoconductive layer is obtained by adding high hardness particles (filler) such as α alumina, titanium oxide, or tin oxide by from 1 percent to 60 percent by weight (wt %) using a dip coating method and a splaying method.
A charging method used to form images using the organic photoconductor includes a corona discharging method that charges the photoconductor with an electrode that is separated from the photoconductor by from about 5 millimeters (mm) to about 10 mm. The charging method also includes a contact charging method of bringing a charging member into contact with the photoconductor. The charging method further includes a non-contact charging method (or proximity charging method) of charging the photoconductor with a charging member that is separated from the photoconductor by from about 30 μm to about 100 μm. A corona charger and a contact charger are generally applied with a direct current (dc) voltage. However, in a case of a non-contact charger or a charger requiring charging stability in particular, a charging member thereof is applied with a voltage by superposing an alternating current (ac) voltage with a voltage of from about 800 to about 2000 volts and frequency of from 600 to 2500 hertz on a dc voltage (450 volts to 850 volts). The function separated type photoconductor is generally negatively charged and a surface voltage thereof is from about −400 volts to about −1200 volts.
A method of visualizing an electrostatic latent image formed on the photoconductor by exposing the image after charging includes a spray-type developing method and a cascade developing method. However, these methods are lack of convenience, and in these days, therefore, a magnetic brush developing method having such advantages as follows is generally used. The advantages are such that downsizing of the image forming apparatus is easy, developing traceability of an electrostatic latent image and high resolution are easily obtained, and a comparatively sufficient signal-to-noise (SN) ratio for background stain is obtained.
Toner used in the magnetic brush developing method often includes pulverized toner whose average sphericity produced by a pulverization method is from about 0.90 to about 0.95 and an average particle size is from about 4 μm to about 10 μm. The pulverized toner has an irregular shape with many irregularities, which allows comparatively better cleaning capability even if a cleaning blade is used.
However, the particle size of the toner used in the magnetic brush developing method is widely distributed (e.g., ±5 μm) and the toner includes many pulverized toner particles. Therefore, charges are difficult to be held identically, and development capability with fidelity to an electrostatic latent image is low, which makes it difficult to obtain sharp edges. Because of this, high resolution is limited. Further, since the charge of the toner is nonuniform, the toner is not fully transferred to a transferred element, which causes much toner to remain on the photoconductor after transfer process, and also causes cleaning failure when micro toner particles of from about 0.5 μm to about 2 μm are included.
The average sphericity is by using FPIA-1000 base on an equation:average sphericity=Σ(circumference of a circle having the same area as a projected area of a particle image÷circumference of a particle projected image”)÷the number of particles measured.It is noted that the number of measured particles is 1,000 or more, particles with a particle size of 5 μm or more are selected, and a toner image is projected to calculate a circumferential length thereof.
The pulverization method is executed by putting additives such as a colorant and a charge control agent into binder polymer produced in a polymerization method, mixing them using a dry type blender, a Henschell mixer, or a ball mill, melting them to obtain a lump, roughly pulverizing and finely pulverizing the lump, and classifying pulverized particles by a sieve or the like for each particle size to produce toner particles.
By mixing 3 wt % to 8 wt % of toner with magnetic powder called carrier such as ion, ferrite, or nickel whose average particle size is from about 40 μm to about 80 μm to cause frictional charging, and the mixture of the toner and carrier is used as developer.
A popular unit of cleaning off powder is a fur blush type unit. More specifically, the powder includes toner and paper dust remaining on the photoconductor after image toner is transferred to a transferred element (paper for Over Head Projector or copy paper). As the fur blush, rabbit fur, pig fur, polyester fabric, or nylon fabric is used conventionally, but currently, a blade cleaning method becomes dominant. The blade cleaning method has advantages in some aspects such as compact size, handling, and manufacturing cost.
A material of the blade used in the blade cleaning method includes an elastic material such as neoprene rubber, chloroprene rubber, silicon rubber, or an acrylic rubber. However, polyurethane rubber (or urethane rubber) is generally used because it does not cause any chemical damage to the photoconductor and has characteristics excellent in durability, ozone resistance, and oil resistance.
The cleaning member of the blade cleaning method using in the cleaning device includes a rubber blade and a support base, and most of cleaning blades are slip-shaped (plate-shaped) cleaning blades each of which thickness is from 1.5 mm to 5 mm.
The cleaning member is used by fixing the slip-cut polyurethane rubber to a metal support such as an iron plate or an aluminum plate using a hot melt adhesive or a double-faced tape so that a free length from the end of the metal support to the edge of the blade is from 2 mm to 10 mm.
The cleaning member is disposed in either manner in which the edge of the blade is directed to the photoconductor in a trailing direction and in a counter direction. Currently, however, the counter method is generally employed because it is excellent in cleaning capability and cleaning maintainability.
The cleaning member is fixed so that the blade edge is in linear contact with the photoconductor and a constant load (contact pressure) of from about 10 g/cm to about 40 g/cm is applied to the cleaning member using a spring or the like. The linear contact is employed in order to avoid excessive frictional resistance between the photoconductor and the cleaning blade, and to make most effective use of the scraping effect by the edge to perform excellent cleaning. Actually, even if the blade edge is in linear contact with the photoconductor, the linear contact is made to be flat and therefore the contact has a width of from about 0.5 mm to about 1 mm. If a contact area becomes wider, toner and paper dust are forcefully pressed against the photoconductor, which is undesirable. For the cleaning performance, therefore, it is desirable to keep the linear contact as much as possible.
The load is applied because the blade edge is brought into tight contact with the photoconductor and a space between them is prevented during rotation of the photoconductor. Therefore, influence of foreign matters existing on or adhered to the photoconductor, irregularities, micro scratches, and of flaws produced when the blade slides along the photoconductor is avoided to keep cleaning capability of the residual powder at a predetermined level.
The cleaning blade is in contact with the photoconductor in the counter direction to cause the blade edge to be engaged in the photoconductor. Accordingly, the tight contact between the photoconductor and the blade edge is enhanced, thus improving the cleaning capability much higher as compared with that of the trailing method. However, if the load is applied too heavily, the blade edge is made to be flat, and the contact is made to be face contact. The face contact increases the frictional resistance with the photoconductor, which causes the blade edge to be pulled in the direction of rotation of the photoconductor and to be returned, that is, a stick-slip phenomenon tends to occur. Thus, both the photoconductor and the cleaning blade are easily and greatly damaged.
Recently, images with high quality such as high-definition and high-resolution color images or monochrome images have been required. With this, polymer toner is increasingly used in printers and electrophotographic copying machines. The polymer toner has an almost spherical shape, and further, the size distribution of particles ranges about ±0.5 μm by using a well-controlled manufacturing method for the polymer toner. Therefore, the polymer toner can be uniformly charged and is excellent in developing capability with fidelity to an electrostatic latent image, transfer capability, and color reproduction when images are superposed on each other.
However, when the pulverized toner is used, even if the cleaning method in which the cleaning capability is excellent because of the contact in the counter direction is used, there comes up such a problem that cleaning is failed at the first sheet if almost spherical toner with high average sphericity is used.
Even if the cleaning is perfectly done at the beginning, cleaning failure may suddenly occur in the middle of copying operation. Furthermore, a large number of sheets may be copied without realizing the number in an imaging device because it performs bulk copy of data at a high circumferential speed.
Substantially spherical toner particles rush to the blade as if they roll over the photoconductor, and therefore, the toner particles slide into even small spaces to easily cause cleaning failure.
During charging to the photoconductor, a large amount of corona product materials (ozone, NOx, or SOx) is produced from the charger to be adhered to the photoconductor. During development, toner is adhered to the photoconductor, and paper dust is adhered thereto during transfer. If a contaminant including the corona product materials, toner, and paper dust adhered to the photoconductor is pressed against the photoconductor by a contact member such as the cleaning blade and the charging member, a film of the contaminant (e.g., toner filming) is formed on the surface of the photoconductor, which causes frictional resistance to increase.
Generally, the polyurethane rubber is used for the cleaning blade so that the blade edge comes in linear contact with the photoconductor. However, if the frictional resistance increases, frictional heat is produced between the cleaning blade and the photoconductor, which causes the film on the surface of the photoconductor to be melted or toner deposited on the blade to be fused. Slidability is thereby degraded, and mechanical pressure balance between the edge of the cleaning blade and the photoconductor is lost. Furthermore, the cleaning blade cannot come in uniform contact with the photoconductor, micro-vibrations are produced with rotation of the photoconductor, and a space between the cleaning blade and the photoconductor is easily produced.
Then, the stick-slip phenomenon occurs, and when the blade edge is pulled at maximum, a further larger space is produced. The stick-slip phenomenon becomes worse with an increase in the frictional resistance of the photoconductor.
Since the frictional force of the blade edge against the photoconductor increases, the photoconductor is easily flawed. Further, visible scratches occur at a portion against which the blade edge is partially and heavily pushed, that is, the surface roughness is caused to increase.
The blade edge is susceptible to damage when the cleaning blade slides along a photoconductor especially including an outermost surface layer that contains a filler of particles with high hardness such as alumina or tin oxide. Specifically, the particles each with a primary particle size of from about 0.1 μm to about 0.7 μm are often used. The agglomeration of the scraped filler is pressed against the photoconductor by the cleaning blade to cause the photoconductor to be deeply scratched and the blade edge to be chipped. This tendency is getting worse with larger particle size of the contained filler.
Furthermore, the photoconductor is hard to be worn, and therefore, the film is easily formed thereon, thus the photoconductor is scraped non-uniformly. Therefore, the frictional resistance of the photoconductor largely increases to cause the blade edge to be deformed or the stick-slip phenomenon to easily occur.
If the deep scratch has been produced, the blade edge is partially twisted or partially applied with pressure, which causes the blade edge to chip.
If the scratch on the photoconductor and the chip of the blade edge become larger, cleaning failure of toner more easily occur.
If the frictional resistance of the photoconductor increases, strong pressure is applied to the blade edge, which causes the blade edge to be partially distorted, resulting in being chipped. A largely chipped part sometimes extends from 120 μm to 200 μm.
If the chip is large, the space between the photoconductor and the cleaning blade is quite impossible to be shielded even if a higher contact pressure is applied. Cleaning failure thereby occurs, and spot-shaped cleaning failure occurs in the initial stage at a portion where the blade largely chips, and the spot-shaped cleaning failure becomes band-shaped. Furthermore, cleaning failure is thinly and widely spread over a portion of the photoconductor where surface roughness is high.
Patent documents that describe frictional resistance between the photoconductor and the cleaning blade are as follows.
Japanese Patent Application Laid Open (JP-A) No. 2000-162802 discloses that an increase in frictional resistance on the surface of a light receiving member speeds up degradation of a cleaning blade and reduces cleaning capability of residual toner to cause cleaning failure to occur.
JP-A No. 2001-1421371 discloses that a cleaning blade is excellent in elasticity, but because of high frictional resistance on the surface of a photoconductor, the edge of the cleaning blade is folded in the direction of rotation of a photoconductive drum, so-called “curling” occurs. This occurs depending on a correlation between pressure force against the photoconductive drum and frictional force with the photoconductive drum, which does not allow normal cleaning.
JP-A No. 2001-265039 discloses that an organic photoconductor has high frictional resistance with respect to a cleaning blade used to remove residual toner, and therefore, the organic photoconductor is worn or the surface of the photoconductor is damaged when the cleaning blade cleans the surface thereof.
JP-A No. 2001-066963 discloses that frictional resistance between a photoconductor and a cleaning blade increases during cleaning to cause the blade to be easily reversed.
JP-A No. 2002-258666 discloses that a frictional coefficient of a photoconductor increases and frictional resistance between cleaning members increases, which causes micro-vibrations or twist of the cleaning member to easily occur on the surface of the cleaning member and cleaning failure of toner to easily occur. As a result, abrasion of a photoconductive layer is speeded up to shorten the life of the photoconductor.
Means of improving cleaning failure of highly spherical polymer toner using the blade cleaning method include the following conventional technologies.
For example, JP-A No. 2001-312191 (Scope of claims, Paragraph Nos. [0012] to [0014], [0067] to [0074], and [0118]) discloses that toner having a shape factor SF-1 of 100 to 140 and toner having a shape factor SF-2 of 100 to 120 are used, a linear pressure of a cleaning blade is set to 20 g/cm or more and less than 60 g/cm. Chips scraped (agglomeration of fluororesin or the like) from the surface of a photoconductor (containing 10 wt % to 50 wt % of fluororesin) are collected to the blade to allow sufficient cleaning to be performed on even highly spherical toner. This is because, by setting a contact pressure of the cleaning blade to slightly higher, it is prevented to form a space between the photoconductor and the blade. By causing the blade to contain a further amount of fluororesin, a frictional coefficient is decreased and the fluororesin is made easier to be scraped. Further, the scraped fluororesin is agglomerated at a place for cleaning by the blade to form a blockage by the agglomerated fluororesin so that the toner is prevented from sliding into the space and cleaning failure is also prevented.
JP-A No. 2001-312191 also discloses in its first example that 30 wt % of fluororesin is added to a surface layer of the photoconductor and the contact pressure (linear pressure) is set to 33 g/cm to perform image formation. However, the frictional coefficient of the photoconductor is kept at a low level because of a large amount of addition of fluororesin, but the quality of a film is friable. Therefore, if the contact pressure is set to 33 g/cm that is higher than ordinary contact pressure, a fluororesin layer is easily worn. As a result, it is found that the durability of the photoconductor is decreased to about one half the durability of a photoconductor without the fluororesin layer. The large amount of addition of fluororesin causes surface roughness (10-point average roughness RzJIS) to be higher than its initial stage by from 2 μm to 3 μm. Accordingly, the surface roughness is increased using the photoconductor for image formation.
With the increase in the surface roughness, corona product materials produced by charging slide into “valleys” of the surface of the photoconductor. Consequently, some part of the blade edge is easily distorted, and at about the same time, the stick-slip phenomenon tends to easily yet gradually occur. The scraped fluororesin is agglomerated at the edge of the cleaning blade, but spherical toner is easy to pass through a fluororesin agglomeration. Therefore, there is some discouraging factor against cleaning failure that may occur with deformation of the blade edge.
JP-A No. 2000-075752 (Scope of claims, Paragraph Nos. [0009] and [0026]) discloses that toner whose shape factor SF-1 is 100 to 140, a cleaning blade whose hardness is from 60 to 80 degrees, and a linear pressure is set to from 55 g/cm to 105 g/cm to perform image formation while applying a lubricant.
In JP-A No. 2000-075752, if spherical toner is used, it is more effective to increase the linear pressure of the cleaning blade as compared with the case where pulverized toner having low sphericity (shape factor is low) is used. However, since the linear pressure in this case is twice to four times higher than the ordinary case, which is abnormally high, a workload to the photoconductor and the cleaning blade become extremely heavy. Therefore, the photoconductor and the edge of the cleaning blade are damaged, and cleaning failure inevitably occurs early because of distortion of the blade edge and the stick-slip phenomenon.
JP-A No. 2002-149031 (Scope of claims, Paragraph Nos. [0025] to [0030]) discloses that cleaning failure is prevented even for substantially spherical toner by making the surface of an image carrier (photoconductor) contain 10 wt % to 50 wt % of fluororesin, and by setting surface roughness Rz of the photoconductor to Rz<5.0 μm, a dynamic frictional coefficient p between the photoconductor and a cleaning blade to 0.5≦μ≦2.5, and a linear pressure A to 200×10−3N/cm<A<600×10−3N/cm.
In JP-A No. 2002-149031 as is disclosed in JP-A No. 2001-312191, by making the photoconductor contain a large amount of fluororesin, the dynamic frictional coefficient is lowered and a contact pressure of the cleaning blade is set to high to improve the cleaning capability of the spherical toner. It is assumed that Rz<5.0 μm is set because the photoconductor is made to contain a large amount of fluororesin, which causes the surface roughness of the photoconductor to become inevitably high.
Surely, by adding a large amount of fluororesin (e.g., Teflon: trademark) to the photoconductor, the dynamic frictional coefficient can be lowered. Consequently, the blade edge is less distorted, and probability of occurrence of cleaning failure is decreased. However, the photoconductive layer is worn abnormally, durability of the photoconductor is largely decreased, and the surface roughness of the photoconductor is made higher and higher. Therefore, the cleaning failure of toner tends to occur early. If the contact pressure (or linear pressure) of the blade is increased in order to recover the cleaning failure, the photoconductor and the blade edge are getting worse and worse to reach a level where the cleaning failure is impossible to be recovered.
Particularly, if the surface layer of the photoconductor has the content of fluororesin with which the dynamic frictional coefficient is kept at such a high level as 2.5, the distortion of the blade edge and the stick-slip phenomenon surely easily occur, and deposition of the corona product materials on the photoconductor causes the dynamic frictional coefficient to be increased, and therefore, cleaning failure may occur permanently.
JP-A No. Hei 11-249328 (Scope of claims, Paragraph No. [0006], FIG. 1) discloses that a layer of a light receiving member is formed with silicon atoms as a base in which frictional resistance of the surface of the photoconductor ranges from 0.1 gram-force (gf) to 150 gf, which allows blade chattering due to friction to less occur and degradation of the blade to be suppressed. It is thereby possible to obtain excellent cleaning capability and increase the variety of toner to be used.
Frictional resistance is measured by a dynamic distortion measuring device produced by HEIDON under the conditions as follows. An elastic rubber blade having a width of 5 centimeters and Japanese Industrial Standards (JIS) hardness ranging from 70 degrees to 80 degrees is pressed at a pressure of 20 g/cm against the photoconductor through a developer mainly containing styrene whose average particle size is 6.5 μm. Under such situations, the light receiving member is made to move at a speed of 400 mm/sec.
In JP-A No. Hei 11-249328, a material used for a photoconductive layer allows satisfactory cleaning. The material contains non-single crystal containing silicon atoms as a base with hydrogen atoms and carbon atoms, or non-single crystal hydrogenated carbon film. Such a photoconductor has high hardness, unlike the organic photoconductor, is extremely dense, and has a surface roughness of 0.1 or lower which is highly smooth. Accordingly, the photoconductive layer is worn extremely less, is never affected by the surface roughness for a long term, and has such durability that image formation of a million sheets or more as the A4-size paper can be achieved. Therefore, there hardly occurs cleaning failure due to surface roughness of the photoconductor or cleaning failure due to largely chipped blade edge. Furthermore, the frictional resistance in the initial stage is low.
Although the photoconductor has the non-single crystal or the non-single crystal hydrogenated carbon film formed on the outermost layer thereof, the photoconductor has a high hardness, and the corona product materials such as ozone and NOx produced during charging are easily deposited thereon, but the photoconductor is hard to be worn. Therefore, the corona product materials are not worn to gradually accumulate thereon, which causes frictional resistance to be gradually increased. As a result, the blade edge is easily distorted and cleaning failure easily occurs caused by micro-vibrations of the blade edge or the stick-slip phenomenon.
The photoconductor described in JP-A No. Hei 11-249328 does not obtain effects by externally adding powdery lubricant such as fluororesin even if the corona product materials are adhered to the photoconductor to cause the physical property of the surface to change. This is because the photoconductive layer is hard and the powdery lubricant is not rubbed into it, unlike the organic photoconductor. In other words, it is difficult to lower the frictional resistance on the surface of the photoconductor, and it is also quite hard to improve the cleaning failure by lowering the frictional resistance with the lubricant.
Although a numerical range of the frictional resistance on the surface of the photoconductor is described in JP-A No. Hei 11-249328, the frictional resistance is largely different depending on measuring units.
Frictional resistance is measured by a dynamic distortion measuring device produced by HEIDON under the conditions as follows. An elastic rubber blade having a width of 5 centimeters and Japanese Industrial Standards (JIS) hardness ranging from 70 degrees to 80 degrees is pressed at a pressure of 20 g/cm against the photoconductor through a developer mainly containing styrene whose average particle size is 6.5 μm. Under such situations, the light receiving member is made to move at a speed of 400 mm/sec.
By setting the frictional resistance to an appropriate range, It is possible to improve the cleaning capability. However, an a-Si photoconductor is different in the physical property on its surface from that of the organic photoconductor. Therefore, the described numeral range is not applied to the organic photoconductor as it is. Furthermore, the measuring method is different from the method described in the present invention.
The a-Si photoconductor is affected by ozone and low-resistance SiO2 is thereby easily formed. Therefore, the frictional resistance on the surface layer of the photoconductor tends to be increased step by step, which may result in going out of the specified range of frictional resistance during using it.
JP-A No. 2001-005359 (Paragraph No. [0040]) teaches to clean the toner using a cleaning blade while applying a solid lubricant to a photoconductor through a brush roller in contact with the photoconductor.
According to the example in JP-A No. 2001-005359, however, as a result of image formation by using toner whose average particle size was 7.5 μm, cleaning failure occurred after image formation of about 23,000 sheets. When the blade edge was checked after image formation of 25,000 sheets was finished, it was observed that the edge of the cleaning blade had a broken (chipped) part with a depth of from 10 μm to 30 μm and a width of from 10 μm to 120 μm. However, only the results were described, and no mention was made of the relation between the depth or the width of the blade and the cleaning failure.
In other words, it is described in JP-A No. 2001-005359 that the solid lubricant was used as a lubricant but there is no description about the numerical values of the frictional resistance or the frictional coefficient. The size of the chipped part of the blade edge is an important factor of the cleaning failure, but the cleaning failure is largely affected by the frictional resistance, and therefore, it is also necessary to define the frictional resistance.
Although it is described in JP-A No. 2001-005359 that the cleaning failure occurred when the chipped part of the blade edge had a depth of from 10 μm to 30 μm, it is presumed that the frictional resistance was extremely high, and so more careful examination on this matter is needed.
The result is that it is important not to produce any factors to cause cleaning failure in order to perform sufficient cleaning of highly spherical toner. The surface roughness of the photoconductor, the frictional resistance, and the surface roughness of the blade edge are extremely important factors. In other words, formation of any space between the cleaning blade and the photoconductor is prevented so as not to pass the toner through the space.
JP-A No. Hei 8-044245 discloses a method of measuring torque of a photoconductor or measuring torque of a rotor in contact with the photoconductor. More specifically, this method is a method of bringing an elastic material such as blade-shaped urethane into contact with the photoconductor with no toner thereon to measure torque applied with load when the photoconductor is made to rotate. Although this method is one of methods effective in measurement of frictional resistance, it has a problem such that the measurement is not stable because the photoconductor is loaded quite heavily. Furthermore, this method is different from the measuring method in the present invention, and measured values are not described in the disclosed method.
If the frictional resistance between the photoconductor and the cleaning blade increases, the stick-slip phenomenon tends to occur. For example, toner produced by the pulverization method or produced by the polymerization method is hard to be cleaned off, which results in degradation of quality of an image on a copied sheet, that is, background stains appear on the image. More specifically, the toner produced by the pulverization method indicates irregular-shaped toner particles having an average sphericity of from about 0.91 to about 0.94 including particles of from about 1 μm to about 3 μm. The toner produced by the polymerization method indicates large spherical toner particles having an average sphericity of from about 0.98 to about 1.0.
Since an engaging force of the cleaning blade to the photoconductor increases, the surface of the photoconductor is damaged, and 10-point average roughness RzJIS as the surface roughness and its maximum height Rz increase, which causes uneven streaks or the like to occur on an image. Furthermore, since the engaging force increases, abrasion of the photoconductive layer is speeded up, which causes scratches to occur and the surface roughness to increase. It is thereby difficult to maintain durability of the photoconductor, and therefore, the life becomes shorter.
The engaging force causes the cleaning blade edge to be worn or easily chipped, streak-like cleaning failure to occur, and overall cleaning failure to easily occur.
The adhesion of the corona product materials to the photoconductive layer is suppressed. Therefore, they are not removed, and a surface frictional resistance rate on the surface layer of the photoconductor lowers, which causes degradation of image quality such as image flow to easily occur.
Since the corona product materials are adhered to the cleaning blade, the blade edge is easily hardened caused by its chemical degradation and easily chipped. The life of the blade is shortened and cleaning failure occurs, which causes streak patterns to easily occur on an image.
The increased engaging force may cause a drum to make unpleasant so-called squeaking noise.
As explained above, if the frictional resistance between the photoconductor and the cleaning blade becomes high, various problems occur. The image quality is thereby degraded, and the life of both the photoconductor and cleaning member is also shortened.